In the recently quickly developed semiconductor production industry, various kinds of gases are used. Of these gases, large amounts of silane series gases such as silane, disilane, tetraethoxysilane, dichlorosilane, or trichlorosilane are used as typical gases.
These gases are usually used in diluted states. However, since these gases are active gases which are combustible and have toxicity, it is necessary for exhaust gases after being used in a semiconductor production step, etc., to be subjected to a cleaning treatment. Also, because some of the gases discharged from a semiconductor production process contain a large amount of silica fine powder which is a decomposition product in the process, in addition to the silane series gases, the removal of the silica which is fine powder must be considered in performing exhaust gas treatment.
The cleaning treatment of silane series gases includes a wet-type cleaning method utilizing a hydrolysis reaction, a dry-type cleaning method utilizing a dehydrogenation reaction, and a combustion-type cleaning method utilizing an oxidation reaction. However, in these methods, the silica which is fine powder contained in the exhaust gas becomes a factor causing various problems.
That is, the wet-type cleaning method involves the problem that the pipes are clogged and a large amount of a slurry is formed, which will require much time to maintain the equipment. Also, the dry-type cleaning method involves the problem that the surface of a cleaning agent is covered with silica, and as a result, the cleaning capacity is decreased and cannot exhibit the sufficient capacity. Furthermore, the combustion-type cleaning method involves the problem that a combustion burner is clogged with the silica fine powder.
Accordingly, it has been desired to efficiently remove the silica fine powder existing in the exhaust gas in a large amount prior to the cleaning treatment of the exhaust gas.
A filtration method is generally used as a method of removing relatively fine powder contained in large amount in an air current. However, the filtration method has the disadvantage that pressure loss at the filter membrane is gradually increased with the passage of filtration and finally it becomes impossible to carry out the filtration. The pressure loss is usually proportional to the thickness of a layer of fine powder accumulated on the surface of the filter membrane. Accordingly, to maintain less pressure loss, the larger the area of the filter membrane, the better. However, to obtain a large filtration area with a flat filter membrane involves the disadvantage that the apparatus becomes large. A technique of pleating the filter membrane is known as a method for avoiding large-sizing of the filter and at the same time increasing the area of the filter membrane. For example, a pleated filter membrane wherein the value obtained by dividing the area of the filter membrane by the external surface area of the filter element is 10 or more is generally used.
However, even in such a filter element having an increased area of the filter membrane, the pressure loss of the filtration film is increased by fine powder accumulated on the surface of the filter membrane with the progress of the filtration, and finally it becomes impossible to continue the filtration. Accordingly, it is necessary to clean or exchange the filter element.
The maintenance of such a filter is troublesome, and various mechanisms of automatically cleaning and regenerating the filter are proposed. For example, a method of shaking down fine powder by applying an oscillation or an impact to the surface of the filter membrane, a method of automatically shaking down fine powder by a brush, scraper, etc., and a back wash method (pulse jet system) of disposing a Venturi tube in the inside of a filter element such as a bag filter, and blowing down fine powder by intermittently jetting a compressed gas from the Venturi tube are known.
In these regenerating methods of a filter membrane, the method of applying an oscillation or an impact to the surface of the filter membrane is an effective method for the filtration of a dried powder having a large bulk density, such as an ore powder, but is not effective for the filtration of a powder having the properties that the specific gravity is small, the bulk density is small, and bridges tend to form among the powder particles, such as the powder contained in an exhaust gas discharged from a semiconductor production step. In particular, in the case of silica formed by the decomposition of tetraethoxysilane contained in the exhaust gas from a semiconductor production step, the fine powder of the silica is fine and have a very strong attaching property, and hence a sufficient effect is not obtained by the method.
The method of scraping down attached fine powder by a brush, a scraper, etc., is also conventional. However, this method involves problems that the structure of the apparatus becomes complicated, and when the fine powder is fine and has a strong attaching property such as silica as described above, it is difficult to completely scrape down and also the filter membrane is injured.
The back wash method of releasing the attached fine powder by jetting a compressed gas from the inside of the filter element is widely used. However, this method has the problem that where the filter element is a filter membrane having complicated pleated form, it is difficult to sweep away the accumulated fine powder at the portion where the filter membrane is adjacent to each other with a narrow space, such as the folded portion, etc., of the pleat.
On the other hand, when the form of the filter element is a relatively simple bag filter, the fine powder accumulated on the filter membrane can be blown off by a back wash method of intermittently jetting a compressed gas every few minutes while performing the filtration. However, even in this method, when the cycle of the back wash is long, the accumulated fine powder is compressed to increase the attaching property, which decreases the blowing off effect.
In particular, in a single wafer processing CVD apparatus wherein a large number of wafers are inserted in a processing apparatus and continuously processed in the apparatus, the processing time per one batch is very long and an allowable value of the pressure fluctuation in the apparatus is very small. Therefore, a continuous filtration of a long time is required. Further, the back wash method of jetting a compressed gas while performing a filtration has a disadvantage of causing a pressure fluctuation, and for this reason, the back wash method cannot be employed. Thus, when the cycle of the back wash is prolonged and the back wash is carried out in a batch unit of wafer feeding, there is a problem that fine powder forms bridges and thus the fine powder cannot be sufficiently blown down. In particular, because the silica formed by the decomposition of tetraethoxysilane is the fine powder fundamentally having a very strong attaching property, there is a problem that when the cycle of the back wash is prolonged, the effect of blowing down the fine powder is not obtained.
Thus, it has been desired to develop a dust removing apparatus and a dust removing method capable of removing with a high efficiency the fine powder having a high attaching property discharged from CVD semiconductor production apparatus without adversely affecting the CVD semiconductor production apparatus system.